This invention relates to direction finding (DF) systems and more particularly to systems for finding the angle of arrival of radio frequency signals received from a transmitter.
DF systems have receiving antennas located at a DF site for receiving a target signal from a transmitter located at some bearing angle from the DF site. DF systems determine the angle of arrival (AOA) of the target signal at the DF site but typically do not determine the distance between the DF site and the transmitter.
Geolocation systems find the geolocation of a transmitter within a region. By knowing the location of the transmitter in the region, a geolocation system can determine the angle of arrival of the target signal at any receiving antenna in the region and can determine the distance between the transmitter and the receiving antenna.
Geolocation systems determine the location of a transmitter using two or more antennas at spatially separated antenna sites. Two DF systems can determine the location of a transmitter by finding the intersection of the lines from the DF system at the measured AOA's. Geolocation systems can use time difference of arrival (TDOA) of signals from the transmitter to the antennas to determine the geolocation of the transmitter. TDOA systems require three or more sites to be highly synchronized in time (to less than 10 nsec), require a broadband network to transfer signals to a central processor, produce large errors when the main signal is blocked by a building or something else and produce very large errors if the transmitter is not within the triangle defined by the three sites.
DF systems in general are simpler and less expensive than geolocation systems. DF systems are of two main types, phase-comparison and amplitude-comparison.
In phase-comparison DF systems, two or more antennas are configured so that the relative phases of the antenna outputs are unique. The angle of arrival is computed by analyzing the relative phases of the antenna outputs. Interferometer systems are examples of phase-comparison DF systems. Interferometer systems have relatively high angle of arrival (AOA) measurement accuracy (1°-3°), but their costs are high and extensive phase calibration is required. The interferometer systems must be thermally calibrated or compensated and multiple antennas are needed for each band to resolve ambiguities. For lower frequencies, the antenna arrays can get quite large.
In Watson-Watt systems, two co-located sets of antennas are oriented perpendicularly and the signals from the sets of antennas are subtracted. The bearing angle is computed by taking the arctangent of the ratio of the subtracted signals. The subtraction is done at RF and requires the phase of the two antennas, cables and subtraction circuit to be well matched. This operation inherently makes Watson-Watt systems phase sensitive. Watson-Watt systems are inexpensive and compact with medium AOA accuracy (4°-7°), however, they usually only cover up to 1 GHz.
In amplitude-comparison DF systems, two or more unique amplitude outputs are obtained from one or more directional antennas positioned at different angles. The angle of arrival is computed by analyzing the relative amplitudes of these outputs.
The antennas in DF systems are connected to one or more radio receivers. The radio receivers have various configurations including single-channel and multi-channel radio receivers. The antenna systems include, for example, configurations such as (i) a single-channel receiver connected to a single antenna, (ii) a single-channel receiver connected to multiple antennas and (iii) a multi-channel receiver connected to multiple antennas.
In one example of an amplitude-comparison DF system, a single antenna for receiving the transmitter signal is attached to a rotating pedestal and is connected to a single-channel receiver. The angle of arrival of the transmitter signal is determined as the pedestal angle where the received signal is strongest.
In another example of an amplitude-comparison DF system, a multi-antenna array is connected to one or more receivers for receiving the transmitter signal. The power levels of the signals received by the different directional antennas are used to determine the angle of arrival of the transmitter signal. With well selected antennas, the power levels of the received signals at the strongest two adjacent antennas in a multi-array of antennas are unique for determining the angle of arrival of the signal.
The multi-antenna array systems using amplitude comparison of the power level have the following advantages over other DF systems: no phase calibration/stability requirements; no precise timing requirements; inexpensive; one set of antennas can cover a wide frequency range; simple power calibration; the package containing the system can be light weight and compact; insensitivity to aging and thermal issues since these effects tend to be common mode to all antennas and hence are cancelled in the ΔPower calculations.
The multi-antenna array systems using amplitude comparison of the power values have the disadvantages of low accuracy and high sensitivity to RF local field amplitude variations caused by multipath and other problems.
Some direction finding systems are mobile and are located on vehicles. As the vehicles move, the systems calculate the angle of arrival of received signals. To do so, the angle of arrival outlier measurements are discarded and the rest of the measurements are processed to calculate a weighted mean angle of arrival. Some direction finding systems are stationary and operate to determine the angle of arrival of a stationary transmitter.
While many amplitude-comparison DF systems have been employed, the systems suffer from poor accuracy or high sensitivity to multipath interference and non-ideal antenna and system characteristics. Many of these problems cannot be corrected by calibration. While such amplitude-comparison DF systems are inexpensive, compact and relatively free from thermal and phase problems, they generally have low AOA accuracy (10°-15°) where higher AOA accuracy of from 1°-3° is needed.
In consideration of the above background, there is a need for improved multi-antenna array amplitude-comparison DF systems with higher accuracy and with reduced sensitivity to problems which cannot be corrected with calibration.